1. Field of the Invention
The present invention relates to a rare earth permanent magnet powder which exhibits superior magnetic properties, particularly anisotropy and corrosion resistance, a method for producing the same, and bonded magnets produced using the above rare earth permanent magnet powder.
2. Prior Art
The rare earth-iron-boron alloy permanent magnet powder, comprising iron (Fe), boron (B) and a rare earth element (which is now defined to include yttrium (Y) and will be hereinafter represented by R when appropriate), has drawn general attention as a permanent magnet material which exhibits superior magnetic properties, and it has since been in the development stage as a magnet powder for bonded magnets. Although a bonded magnet is inferior in magnetic property when compared to sintered magnets of the same magnet powder, its field of application has grown and continues to grow rapidly in recent years because of its superior physical strength and higher flexibility in shape. These bonded magnets are composed by binding magnet powder with organic binders and/or metallic binders, or the like; and magnetic properties of these bonded magnets depend on the magnetic properties of such magnet powders.
Japanese Patent Application A-Publication No. 1-132106 describes one prior art R-Fe-B permanent magnet powder used to manufacture a bonded magnet. The R-Fe-B permanent magnet powder is manufactured by preparing a R-Fe-B alloy having a ferromagnetic R.sub.2 Fe.sub.14 B intermettalic compound phase as its principal phase, subsequently heat-treating the alloy in a prescribed temperature range in a hydrogen atmosphere to cause phase transformation in RH.sub.x phase, Fe.sub.2 B phase and the remaining Fe phase, and subsequently subjecting the alloy to dehydrogenation to reproduce the R.sub.2 Fe.sub.14 B phase. The resulting magnet powder has an aggregated structure of recrystallized grains of R.sub.2 Fe.sub.14 B phase having a minuscule average grain size of 0.05 .mu.m to 3 .mu.m.
Japanese Patent Application A-Publication No. 1-132106 further describes a R-Fe-Co-B permanent magnet powder, i.e., a R-Fe-B magnet powder in which a part of Fe is substituted by cobalt (Co). This magnet powder has also an aggregated structure of recrystallized grains of R.sub.2 (Fe,Co).sub.14 B phase having an average grain size of 0.05 .mu.m to 3 .mu.m.
The aforesaid prior art magnet powders exhibit magnetic anisotropy, but anisotropy is often reduced due to small fluctuations in alloy composition or manufacturing conditions, and hence it is difficult to obtain stable and superior magnetic anisotropy.
Known methods for imparting magnetic anisotropy to a magnet powder involve subjecting the permanent magnet powder to hot plastic working, such as hot rolling and hot extrusion, to thereby form the crystal grains of the magnet powder into a flat shape. In such a hot plastic working, however, the degree of working varies depending upon the locations, and hence a magnet powder having a stable and uniform magnetic anisotropy cannot be obtained. In addition, the manufacturing process is intricate, and hence the manufacturing costs are unduly high.
Furthermore, the flat recrystallized grains formed by means of hot plastic working are more susceptible to corrosion than the magnet powder comprising recrystallized grains which are not hot-worked. As a result, when the magnet powder is kept in a high-temperature and high-humidity atmosphere, such as in a factory, for a prolonged period of time, the surface of the magnet powder is corroded, and its magnetic property is unduly reduced.